Android & Apple Messaging Notifications Setup

Hi, I’m looking for a freelancer who can help implement push notifications for my existing app.

Currently, my app only supports in-app notifications, so users can only receive notifications about new messages when they have the app open. I want to implement push notifications so users can receive alerts when someone sends them a message, even when the app is running in the background or closed.

I’d like to know:

If you can implement this feature in my existing app.
What technology or push notification service you would recommend.
How much you would charge for the full implementation.
How long it would take to complete.
Whether there are any additional ongoing costs or service fees.

Please let me know your estimated price and what information you need from me to assess the project.

Thank you!